How Much Force can a Coupler Withstand?

Actual Practices

When running trains in the 1980s with 5 SD40-2s I found out the hard way you can shear perfectly good drawbars (no slipping or jerking, just pulling) by going from #4 to #5 throttle while stopped and trying to start a train on a hill. #4 is OK. #5 is NOT. It will break almost everytime. (I am a slow learner, it took me 3 successive train break in twos to realize DON'T DO THAT).
Now our 420,000 lbs SD40-2s are capable of about 90,000+ lbs of TE each. So 5 times that would be 450,000 lbs. Standard Grade E drawbars and knuckles are NOT good for 450,000 lbs!

Occasionally I will get 4 SD70MACS or combos of other high Ad units. One day I had 2 UP C44ACs, a UP SD90/43, and a BNSF SD70MAC. This combo is illegal on BNSF because we are limited to 36 powered axles. The MACs are rated at ELEVEN power axles each so 3 = 33 axles and 4= 44 axles. Clearly 4 of these units is illegal under the rules. But you have to try it at least once don't you? As we got slower and slower on the hill I got more and more chicken. When the tractive effort meter reached 96,000 lbs (4 x 96,0000 = 384,000 lbs) I notched off one and went up the hill in #7 throttle. I probably could have left them in #8 but I just figured the gamble was not worth the potential delay. I have heard other engineers say they have gone up the hill with 4 biggies on the point in #8 with no problems. But to me you are pushing your luck and if it does separate you are going to have some real explaining to do as to why you had 4 high HP units on line.
(BNSF has since revised the powered axle limitation upward a bit but it still does not permit 4 hi-HP units).

It always breaks 10 Cars Back

• First is that if locos are slipping badly they will unload temporarily and allow the slack to roll in on 10-15 cars. A few seconds later when the locos re-load they stretch that slack out from the front towards the back. That means when the slack is taken up on that last car, usually somewhere in the 10th thru the 15th car, you have a solidly stretched 2000+ tons traveling at a slightly greater speed than the remaining solidly stretched 14,000 tons of the train. While the locos are taking up the slack on those first 10 or 15 cars they are jerking only one car at a time to equalize the speed and jerking one car at a time 1 or 2 mph is no big deal. But when the stretching of the slack gets to that last bunched car......believe me when two solid blocks of multi-thousand tons going at slightly different speeds yank on that last drawbar SOMETHING is going to give.
• Secondly most break in twos occur at the crest of a grade or the lessening of a grade.
  That is usualy because the hogger did not throttle off enough when topping the hill. Remember that 600-1200 tons of locomotives does not go up the hill for free. Some of their power is used up moving themselves up the hill. That leaves less pull for the train. As the locos top over they no longer require any power so the total output is now available at the first car's drawbar. Worse, as the locos and first few cars start down the otherside their weight ADDS to the total drawbar pull felt by which ever car is at the crest. Three or 4 locos plus 10 cars is about 2,000 tons. On a 1.5% downgrade that is 60,000 lbs additional drawbar pull due solely to their weight on the downgrade. That is why most break in twos occur about 10 cars back in the train. It is because THAT is where the drawbar pull reaches its highest ever figure if the hoghead does not compensate by notching off.
  If you are marginally near the drawbar limit on a straight pull then sometimes even a temporary slight lessening of the grade is enough to tranfer enough additional TE from the locos to the train to break it.
  

Created 06-06-2001
Updated 06-06-2001